The present invention relates to a mechanism for transporting and positioning printing masters in a processing unit, in which the printing masters are transported from a plate holder to an exposure table.
Lettered and imaged printing masters which are directly suitable for printing are manufactured in such a way that the printing plates are first electrostatically charged and then an original is projected imagewise upon the individual plate. After this exposing procedure, the printing master is developed with a developer, fixed and decoated. It is then ready for use in a printing press.
German Pat. No. 2,462,216 discloses an apparatus for the manufacture of printing masters by electrophotographic means, in which the individual printing plate is picked up from a stack in a plate holder by a transport device comprising a transport carriage with reduced pressure-suction means. The transport carriage transfers the printing plate to an exposure platform. Charging of the individual printing plate and transporting of the plate to the exposure platform are combined in a time-saving manner by mounting a corona charging-station for electrostatic charging of the printing plates at the front of the transport carriage. The corona charging-station extends transversely to the direction of motion of the carriage. The carriage has a vacuum plate which is attached to its underside and which is connected with a vacuum pump by a number of holes. When the transport carriage is placed on top of the uppermost printing plate in the plate holder, reduced pressure is generated and the printing plate is drawn by suction to the vacuum plate. The transport carriage is then displaced by means of a motor along two guide rails toward the exposure platform. As soon as the carriage has reached its position above the exposure platform, it is lowered. The reduced pressure in the vacuum plate is then released so that the printing plate is set down on the exposure platform. The exposure platform is also designed as a vacuum plate and, under the action of reduced pressure, the printing plate is firmly urged against the platform.
In this known apparatus, the photoconductive, coated side of the printing plate is contacted by the suction means when the individual plate is lifted from the stack. Although these suction means allow a far gentler transport of the printing plate as compared to conventionally used rollers or webs which are guided over the coated side of the plate, mechanical or physical defects caused by the suction means can still occur on the developed printing master.
U.S. Pat. No. 4,149,798 discloses a transport station for printing masters, which includes a stacking area containing a supply of printing masters. A paper disposal area is provided for the sheets of paper which are interposed between the individual masters in the stack. The printing masters are transported from the stacking area to a conveyor by means of a control mechanism equipped with a number of suction cups. The control mechanism is pivotally attached to a support arm which is connected to a motor via a gear mechanism. Thus, the control mechanism is adapted to perform a lateral sliding movement back and forth. The uppermost printing plate in the stack is picked up by at least four suction cups of the control mechanism. Then the control mechanism swings upwardly and, at the same time, the support arm mechanically approaches the conveyor. When the reduced pressure in the suction cups is released, the control mechanism pivots downwardly and deposits the printing master on the conveyor. A second control mechanism positioned above the sheet of paper lying between the two printing plates, engages the sheet, picks it up and conveys it to the paper disposal area. The conveyor transports the printing plates to an exposure platform which has a plurality of holes on its upper surface and which comprises a lower chamber connected to a vacuum pump by a vacuum line. When the printing plate has been positioned on the surface of the exposure platform, reduced pressure is applied and the plate is thus retained on the platform. Just as in the first-mentioned apparatus, the photoconductive layer of the printing plate is contacted by suction means when the uppermost plate is removed from the stack, and this may lead to mechanical or physical defects on the finished printing master.
In the prior art processing units, in which the printing plates are horizontally removed from a plate holder by a suction means and are transported to and lowered upon an exposure table, the position of the printing plate is usually not adjusted before it lies on the exposure table. This may lead to difficulties, if printing plates of different formats are used, since the suction means which is moved along a fixed path will invariably pick up the printing plates at the same points, irrespective of the plate format. A printing plate varying from the DIN A4 size, for example, is not held by suction along its center line. As a result, it may be difficult to position the printing plate after it has been put down on the exposure table, because different stops must be provided for varying formats.